The 3GPP standard Long-Term-Evolution (LTE) of UTRAN is a system using orthogonal frequency division multiplex (OFDM) standards with frequency-localized allocations.
In the LTE system one main difference to earlier 3GPP releases is the use of wide channels that are shared with users in frequency domain (i.e., frequency division multiplexing). Allocation for one user can vary from one physical resource block (PRB) to maximum number of resource blocks in the channel (e.g. 50 PRB for 10 MHz channel). A physical resource block is the smallest allocable frequency range of the uplink or downlink frequency band lasting a predefined time. E.g., in LTE, a physical resource block is 180 kHz wide and lasts for a 0.5 ms time slot.
LTE may have to be capable of substituting incumbent radio access technologies including Code Division Multiplex Access (CDMA), Wideband CDMA (W-CDMA), and Global System for Mobile communication (GSM)/Enhanced Data rate for GSM Evolution (EDGE) where spectrum allocations and regulatory rules will allow for.
It is therefore of vital interest to operators, that migration e.g. from CDMA or from W-CDMA to LTE is possible without the need for further increasing the number of Base Station sites or the amount or size of antenna configurations.
Hence, for the same traffic profile and demand, LTE link budgets shall match those of CDMA or W-CDMA. Further, LTE link budgets matching conventional traffic profiles and demand must also match LTE link budgets for new applications like down streaming or heavy downloading.
The LTE downlink (DL) link budget is critical for the Physical Downlink Shared Channel (PDSCH) with high data rate services like video or multi-media streaming (e.g. ½ Mbps at the cell edge), and for low data rate services like voice over internet protocol (VoIP) (e.g. 5.9 Kbps or 12.2/12.8 Kbps) at the cell edge.
A straight forward method of improving the LTE DL link budget is to allocate as many PRBs for a critical user equipment (UE)/service as possible and to employ an as robust as possible Modulation and Coding Scheme (MCS). In current Rel'8 scope, this is handled by using the lowest possible transport block size index (TBS index). The TBS index is given from the modulation order and the MCS. The mapping from TBS index to transport block size is given in 3rd generation partnership project (3GPP) technical specification (TS) 36.213, section 7.1.7. A transport block (TB) is defined as data accepted by the physical layer to be jointly encoded.
FIG. 1 shows the maximum number of physical resource blocks (PRB) in dependence of the number of information bits in a transport block according to 3GPP specification 36.213, v8.7.0. The curve is given for the smallest Modulation and Coding Scheme (MCS, transport block size index=0) as it is the most robust MCS and, thus, appropriate for extending the limits of the downlink link budget.
As shown in FIG. 1, the number of PRBs that can be allocated at maximum for a given size of the Transport Block and a given Modulation and Coding Scheme (MCS) is limited and may not reach the full LTE DL bandwidth. From this it is seen that in LTE Release 8, it is not possible to transmit with full DL bandwidth a single low data rate service.
An alternative method of improving the LTE downlink link budget is to “borrow” energy from PRBs that are using less than the nominal enhanced NodeB (eNB) transmission power or are not used at all. This means that we can potentially lower the power allocated to non-critical UEs and/or services. This borrowing is specified and can be at most 3 dB up and at most 6 dB down for PDSCH (see 3GPP TS 36.104 v8.5.0, Section 6.3.1.1).
Generally, such resource element (RE) power boosting will only work, if not all of the PRBs are allocated to the critical UE/service already; otherwise there is no resource to borrow power from.